Unexpected temperature readings during fueling Friday prompted the Air Force and United Launch Alliance to postpone the planned launch of a Delta 4 rocket carrying a classified National Reconnaissance Office satellite. After troubleshooting, launch was rescheduled for 5:58 p.m. EST Sunday, the opening of a presumed four-hour window.

Liftoff from complex 37 at the Cape Canaveral Air Force Station had been targeted for 6:06 p.m. EST Friday. But shortly after fueling began, sensors indicated temperature anomalies in the left and right common core boosters. Fueling was interrupted and engineers began troubleshooting the problem. A fire alarm at the pad went off at one point, but that turned out to be a false indication.

After additional troubleshooting, mission managers decided to call off the countdown and an official scrub was announced.

"During the launch countdown, anomalous temperature data signatures were detected on the port and starboard strap-on common core boosters during cryogenic fueling," United Space Alliance said in a statement. "When detected, mission managers halted the countdown and stopped further tanking of the rocket. After evaluation, the decision was made to scrub today's launch attempt."

As it turned out, the anomalous temperature readings were the result of problems with two sensors. The sensors were replaced and United Launch Alliance announced Saturday that launch had been rescheduled for Sunday. Forecasters are predicting a 90 percent chance of acceptable weather.

The NROL-32 payload is believed to be an electronic eavesdropping satellite with a huge collecting antenna. In a September address to the Air Force Association, NRO Director Bruce Carlson said the Delta 4 was carrying "the largest satellite in the world."

"I believe the payload is the fifth in the series of what we call Mentor spacecraft, a.k.a. Advanced Orion, which gather signals intelligence from inclined geosynchronous orbits," Ted Molczan, a respected satellite tracker, told Spaceflight Now. "They are among the largest satellites ever deployed."

Once on station, the satellite presumably will unfold a huge, lightweight antenna to tap into targeted military or civilian communications networks.

"The satellite likely consists of sensitive radio receivers and an antenna generally believed to span up to 100 meters (328 feet) to gather electronic intelligence for the National Security Agency," Molczan told Spaceflight Now.

The NROL-32 launch was delayed one day, from Thursday to Friday, because of work to replace suspect cables in the pyrotechnic system used to release the huge rocket from its firing stand.